The present invention relates to a silicon single crystal wafer suitable for a substrate of epitaxial wafer used in fabrication of semiconductor devices and an epitaxial wafer utilizing it, as well as methods for producing them, and the present invention further relates to a method for evaluating a silicon single crystal wafer suitable for a substrate of epitaxial wafer.
It is well known that grown-in defects existing in CZ silicon single crystals pulled by the Czochralski method (CZ method) degrade oxide dielectric breakdown voltage characteristics of wafers, cause isolation failures in the device production step and so forth, and various methods are proposed to obviate such problems.
For example, there are a method of reducing grown-in defects during the pulling by the CZ method, a method of eliminating surface defects by subjecting a wafer to annealing at high temperature in a hydrogen or argon atmosphere, a method of using an epitaxial wafer in which an epitaxial layer is grown, and so forth.
Further, as the integration degree of semiconductor devices becomes higher in recent years, it is becoming more important to reduce crystal defects in semiconductors, in particular, crystal defects on and near surfaces thereof. For this reason, the demand for epitaxial wafers in which an epitaxial layer excellent in crystallinity is formed on a wafer surface increases every year.
Meanwhile, when devices are produced by using an epitaxial wafer, various heat treatment steps are usually used in addition to the epitaxial growth. If contaminations such as heavy metal impurities are present during these steps, they will markedly degrade device characteristics. Therefore, such contaminants must be eliminated from the epitaxial layer as much as possible. Accordingly, a substrate having high gettering effect is required as a substrate for the epitaxial growth.
Gettering includes extrinsic gettering (EG) and intrinsic gettering (IG). As typical EG techniques, there are technique of polysilicon back seal in which a polysilicon film is deposited on a back surface of substrate, technique of mechanically damaging the back surface and so forth. However, not only these techniques suffer from problems of particle generation and so forth, but also they are extremely disadvantageous in view of cost since they require special process steps.
On the other hand, in IG, a CZ method silicon wafer containing oxygen is subjected to a heat treatment to generate oxide precipitates that become gettering sites in a bulk portion of substrate. However, in the case of an epitaxial wafer, it suffers from a problem that oxide precipitate nuclei originally existing in a substrate are eliminated during the epitaxial growth at a high temperature, and gettering ability becomes insufficient because oxide precipitates are not likely to be formed and to grow during the subsequent device heat treatment.
Therefore, the conventional production of epitaxial wafers utilizes the fact that a substrate containing boron at a high concentration (p+ substrate) has the gettering effect, that is, there is often used a pxe2x88x92/p+ epitaxial wafer, in which an epitaxial layer of low boron concentration (pxe2x88x92) is formed on a p+ substrate. However, when epitaxial growth is performed on a p+ substrate, there are caused a problem of autodoping, in which boron doped at a high concentration is evaporated from the substrate and taken up into the epitaxial layer during the epitaxial growth, or a problem that boron is taken up from the substrate surface into the epitaxial layer due to solid phase out-diffusion. Further, since demand for epitaxial wafers utilizing pxe2x88x92 substrates is increasing in recent years for CMOS devices, insufficient gettering ability becomes to constitute a problem.
Furthermore, fairly recently, there is frequently seer effective use of techniques utilizing characteristics of crystals doped with nitrogen as methods for obtaining CZ wafers having reduced grown-in defects existing near wafer surfaces. For example, there can be mentioned a technique in which defects in a deeper region are eliminated through annealing by doping nitrogen into crystals to make a size of grown-in void defects smaller in order to enhance easiness of elimination of defects during the annealing at a high temperature, a technique of producing epitaxial wafers showing enhanced IG ability by using crystals doped with nitrogen for substrates of the epitaxial wafers so as to enhance formation of oxide precipitates during the device heat treatment and thus increase BMDs (Bulk Micro Defects) and so forth.
As an example of use of such nitrogen-doped crystals for substrates for epitaxial growth, in the technique described in Japanese Patent Laid-open (Kokai) Publication No. 11-189493, a silicon single crystal grown with doping of nitrogen at a level of 1013 atoms/cm3 or more is used for epitaxial wafers. This technique was based on the finding that, if an epitaxial layer was formed on a substrate containing an OSF (Oxidation induced Stacking Fault) region, which was generated in a ring shape depending on the pulling conditions of single crystal in the CZ method, oxygen precipitation nuclei existing in the OSF ring region were not eliminated, but they functioned as effective gettering sites in the device production process after the epitaxial formation, and the finding that the width of the OSF ring could be made larger by doping nitrogen during the single crystal growth, and if the amount of nitrogen to be doped was 1013 atoms/cm3 or more, the nuclei of OSFs effective for the gettering could be uniformly distributed over the whole single crystal.
However, according to the investigation of the inventors of the present invention, it became clear that, if an epitaxial layer was formed on a wafer doped with nitrogen, defects called LPDs (Light Point Defects, generic term for referring to bright spot defects observed by using a wafer surface analysis apparatus utilizing laser light), which were harmful to devices, were likely to be formed on the OSF region of the epitaxial layer. Further, it was also found that these LPDs were particularly notably observed, when the nitrogen concentration was high. That is, if an epitaxial layer is formed utilizing the technique described in the aforementioned Japanese Patent Laid-open (Kokai) Publication No. 11-189493 as it is, there are likely to be obtained an epitaxial wafer in which many LPDs are generated. Therefore, as a countermeasure for this, decrease in nitrogen concentration is contemplated. However, the decrease in nitrogen concentration results in reduction of two of the intrinsic effects of doping with nitrogen, that is, the reduction in void size, i.e., improvement effect of elimination efficiency of defects by the annealing, and the improvement effect of IG ability brought by the enhancement of oxygen precipitation.
The present invention was accomplished in view of the aforementioned problems, and its object is to provide a substrate for an epitaxial wafer that suppresses crystal defects to be generated in an epitaxial layer when epitaxial growth is performed on a CZ silicon single crystal wafer doped with nitrogen and also has superior IG ability and an epitaxial wafer utilizing such a substrate, as well as methods for producing them. Further, another object of the present invention is to provide a method for evaluating such a substrate suitable for an epitaxial wafer.
In order to achieve the aforementioned objects, the present invention provides a silicon single crystal wafer for epitaxial growth grown by the CZ method, which is doped with nitrogen and has a V-rich region over its entire plane.
Such a silicon single crystal wafer as described above, which is obtained by processing a silicon single crystal ingot doped with nitrogen in the CZ method and has a V-rich region over its entire plane, can suppress crystal defects to be generated in the epitaxial layer during the epitaxial growth, and moreover, it can be a silicon single crystal wafer having a superior IG ability and suitable for epitaxial growth. Therefore, the influences on the device production can be substantially eliminated, and it can be a silicon single crystal wafer that can improve production yield or quality characteristics of devices.
The present invention also provides a silicon single crystal wafer for epitaxial growth grown by the CZ method, which is doped with nitrogen, has an OSF region in its plane, and shows an LEP density of 20/cm2 or less in the OSF region.
The present invention further provides a silicon single crystal wafer for epitaxial growth grown by the CZ method, which is doped with nitrogen, has an OSF region in its plane, and shows an OSF density of 1xc3x97104/cm2 or less in the OSF region.
In such wafers, generation of LPD can be suppressed when an epitaxial layer is formed on the wafer.
In the aforementioned wafers, the nitrogen concentration is preferably 2xc3x971013/cm3 to 1xc3x971014/cm3.
If the nitrogen concentration is 2xc3x971013/cm3 or more as described above, sufficient BMDs can be obtained even after an epitaxial layer is formed, and they act as effective gettering sites in the device production process. Further, if the nitrogen concentration is 1xc3x971014/cm3 or less, the generation of LPDs in the epitaxial layer can be effectively inhibited.
The present invention also provides an epitaxial wafer in which an epitaxial layer is formed on a surface of any one of the silicon single crystal wafers mentioned above.
If an epitaxial layer is formed on a surface of a silicon single crystal wafer in which crystal defects are controlled as described above, a silicon epitaxial wafer of high quality can be obtained, which has very few LPDS, BMDs of sufficient density in a bulk portion of the substrate by the effect of nitrogen doping, and hence superior gettering ability. Therefore, it can be a silicon epitaxial wafer in which IG ability is enhanced by promoting the formation of oxide precipitates during the device heat treatment to increase BMDs.
The present invention further provides an epitaxial wafer, which has 1xc3x97108/cm3 or more of BMDs in a silicon single crystal wafer which is doped with nitrogen and on which an epitaxial layer is formed and a defect density of 0.11/cm2 (20/wafer having a diameter of 6 inches) or less on an epitaxial layer surface for defects having a size of 0.11 xcexcm or more.
The present invention also provides a method for producing a silicon single crystal wafer for epitaxial growth, wherein, when a silicon single crystal doped with nitrogen is grown by the Czochralski method, the crystal is pulled under such a condition that the crystal should have a V-rich region for entire plane of the crystal.
If a silicon single crystal is pulled with nitrogen doping under such a condition that the crystal should have a V-rich region for entire plane of the crystal as described above, a wide control range and ease of control can be attained, and the crystal can be grown at a high speed. Therefore, silicon single crystal wafers for epitaxial growth can be produced with high yield while maintaining high productivity.
In the aforementioned method, as such a condition that the crystal should have a V-rich region for entire plane of the crystal, specifically, V/G (V: pulling rate [mm/min], G: temperature gradient along the direction of crystal growth in the vicinity of solid-liquid interface [xc2x0 C./mm]) during the crystal growth is desirably controlled so that the OSF region should be eliminated from a peripheral portion of the crystal toward the outside.
The present invention also provides a method for producing a silicon single crystal wafer for epitaxial growth, wherein, when a silicon single crystal doped with nitrogen is grown by the Czochralski method, the crystal is pulled so that a center portion of the crystal should become a V-rich region and a peripheral portion of the crystal should become an OSF region, and then the OSF region is eliminated.
Even if the OSF region is not completely eliminated as described above, silicon single crystal wafers suitable for epitaxial growth can be produced by suppressing the generation of OSF by a position at a distance of about 20 mm from the periphery of the crystal toward the center, and removing this region in a subsequent processing of the single crystal ingot.
In these methods, a cooling rate within the temperature region of 1000xc2x0 C. to 900xc2x0 C. during the crystal growth is preferably adjusted to be 0.8xc2x0 C./min or less.
Further, when the silicon single crystal doped with nitrogen is grown by the Czochralski method, the crystal can be pulled so that a center portion of the crystal should become a V-rich region and the crystal should have an OSF region, and a cooling rate of 0.8xc2x0 C./min or less can be used within the temperature region of 1000xc2x0 C. to 900xc2x0 C.
When a crystal having an OSF region is used considering production cost and so forth, if slow cooling is used around the temperature region of 1000xc2x0 C. to 900xc2x0 C. as described above, although the density of OSFs is not changed, the morphology of the OSF nuclei can be changed to inhibit LEPs (Large Etch Pits) to be generated at the positions of OSF in the nitrogen doped crystal, and as a result the generation of LPDs in the epitaxial layer can be prevented.
The present invention further provides a method for producing a silicon single crystal wafer suitable for epitaxial growth, which comprises subjecting a silicon single crystal wafer produced by the aforementioned production methods to an IG heat treatment.
If a silicon single crystal wafer in which crystal defects are controlled before forming an epitaxial layer is subjected to the so-called IG heat treatment as described above, a DZ layer to be formed on the surface of substrate can be more deeply formed by the effect of nitrogen doping compared with a case not utilizing the nitrogen doping. Therefore, the wafer becomes more preferred one for devices and extremely good crystallinity of the epitaxial layer formed on its surface can be obtained.
The present invention further provides a method for producing an epitaxial wafer, wherein an epitaxial layer is formed on a surface of silicon single crystal wafer produced by the aforementioned production methods.
By such a method, there can be produced an epitaxial wafer which has an epitaxial layer of high quality without crystal defects and has enhanced IG ability due to increased BMDs.
Furthermore, the present invention also provides a method for evaluating a silicon single crystal wafer for an epitaxial wafer, wherein wafers are sliced from both ends of a silicon single crystal ingot produced by the Czochralski method with nitrogen doping, and both of the wafers are subjected to preferential etching and then density of LEPs generated on the wafer surfaces is measured, or both of the wafers are subjected to thermal oxidation treatment and then preferential etching and density of OSFs generated on the wafer surfaces is measured, so as to evaluate presence or absence of generation of crystal defects on a surface of epitaxial layer to be grown on a silicon single crystal wafer produced from a remaining portion of the silicon single crystal ingot from which the both wafers were sliced.
If a silicon single crystal wafer for an epitaxial wafer is evaluated as described above, it becomes possible to evaluate the quality at a point before processing of the wafer, and hence processing and epitaxial growth using a defective wafer can be prevented. Therefore, the cost can be markedly improved as the whole process.
As explained above, according to the present invention, silicon single crystal wafers suitable for epitaxial growth can be stably produced with high yield and high productivity by pulling a single crystal with nitrogen doping under such a condition that the crystal should have a V-rich region for entire plane of the crystal, or such a condition that the crystal should have an OSF region in its plane and the region should have a low crystal defect density. Furthermore, if an epitaxial layer is formed on a surface of such a silicon single crystal wafer, there can be easily produced silicon epitaxial wafer having suppressed crystal defects in the layer, and IG ability enhanced by promoting the formation of oxide precipitates during the device heat treatment to increase BMDs.